Optical waveguides can be used to expand or replicate the exit pupil of an imaging system in one or two dimensions. Typically, light from the exit pupil of the imaging system is received in the waveguide through an entrance or in-coupling, and travels through the waveguide in a direction, while some of the light exits a grating structure of the waveguide creating an expanded pupil. The remaining light that does not exit the grating structure is wasted and typically exits or is absorbed at the edges of the waveguide. Current waveguides typically use low-efficiency single pass grating structures to achieve pupil uniformity and field of view uniformity, which results in a large amount of wasted light.
One issue with current waveguide-based exit pupil expanders is they are designed to use incoherent light sources. An incoherent light source is a light source whose light contains a broad range of frequencies and thus a short coherence length on the order of 10 periods. An example of an incoherent light source is an LED. Coherent light sources, on the other hand, are light sources containing a narrow range of frequencies and thus a longer coherence length. Coherent light sources when split into multiple paths will generally interfere with themselves when recombined if the difference in optical path length traversed by each optical path of the light is less than a coherence length associated with the coherent light source. An example of a coherent light source is a laser.
In order to achieve a uniform display intensity of the expanded pupil, the distance between each exit pupil of the waveguide is typically small so that the viewer receives overlapping light from multiple pupils. However, because of the closeness of the pupils, and the similar optical paths of the light received from each pupil, when the waveguide is used with a coherent light source the overlapping light will interfere with itself and may result in a diminished experience for the user.